1. Field of the Invention
The present invention relates generally to the field of heaters for use in analytical or test instruments and, more specifically, to a block-type heater assembly which provides very uniform heating and improved temperature measurement accuracy while allowing observation of samples contained within capillary tubes inserted into the heater assembly.
2. Discussion of the Related Art
In many types of analytical testing, including melting point or boiling point determinations, it is necessary to heat one or more samples (which arm usually placed in containers) in a controlled fashion while allowing observation of the samples in order to record events of interest. Designing a suitable heater for such testing requires the solution of several problems including, among others, providing access for observation of the samples, providing for substantially uniform heating of the samples, providing a way to accurately monitor the temperature of the samples, and providing a mechanism which is inexpensive to manufacture and will reliably operate when subjected to a large number of repetitive tests over many years.
In general, two families or types of heaters 1lave been developed to address the foregoing problems: the oil bath-type heater and the block-type heater. Oil bath-type heaters are generally recognized as providing superior performance in terms of heating uniformity and temperature measurement accuracy. Containers in which samples are held are immersed in a stirred volume of oil. A thermometer or temperature sensor is used to measure the temperature of the oil, which is fairly constant throughout the bath. However, there are several disadvantages which are inherent to such heaters including ability to practically provide for viewing of sample containers once immersed in the oil, temperature the inconvenience and housekeeping related to handling oil, and the relatively high cost of such heaters.
Conventional block-type heaters consist essentially of a block of material, usually a metal such as aluminum, having an embedded or attached heating element The block includes apertures or recesses into which sample-bearing containers may be placed. A thermometer or temperature sensor is usually inserted into or placed in contact with the block to measure temperature While less costly and more convenient to use than oil bath heaters, block-type heaters are generally recognized as providing less uniform heating and reduced measurement accuracy. Both of those disadvantages are typically caused, at least in part, by the geometry of the block, the placement of the sample containers and the temperature sensor relative to each other or the heating element, the necessity of providing access for observation of the samples or a combination of same. Some conventional block-type heaters attempt to compensate for such deficiencies through the use of insulation. That approach is usually unsatisfactory because of limitations in the types of insulation material suitable for use with heaters operating at relatively high temperatures and difficulty in physically placing insulation where needed. In addition, the introduction of insulation creates the potential for trapping contaminants and renders the heater more difficult to clean.